Typically, a "hot wire" type flowmeter includes a self-heated sensor resistor having a resistance Rh which is a function of its temperature. In turn, the temperature of the heated resistor is determined at least in part by the difference between the heat generated within the heated resistor as a function of the voltage applied across the resistor and the heat transferred away from the heated resistor as a function of the amount of cooling fluid flow past the resistor. In addition, it is usual for a "hot wire" flowmeter to include an ambient temperature sensing resistor having a resistance Ra determined by the ambient temperature of the flowing fluid.
In a bridge-type "hot wire" flowmeter, the self-heated resistor and the ambient temperature resistor are connected within a bridge circuit across which a voltage Vb is developed. In terms of fundamental structure, the bridge circuit includes a signal side for deriving a signal voltage Vs which is a voltage divided function of the bridge voltage Vb as determined at least in part by the resistance Rh of the sensor resistor in ratio to the resistance Rp of a power dissipating resistor. The bridge circuit further includes a reference side for defining a reference voltage Vr which is a voltage divided function of the bridge voltage Vb as determined at least in part by the sum (Ra+Rb) of the resistance Ra of the ambient resistor plus the resistance Rb of a ballast resistor in ratio to the resistance Rc of a calibration resistor.
It is common in a bridge-type flowmeter to drive the bridge circuit with an operational amplifier which compares the signal voltage Vs with the reference voltage Vr. More specifically, the amplifier is responsive to the difference between the two voltages Vs and Vr to alter the bridge voltage Vb thereby correspondingly altering the voltage applied across the self-heated resistor so as to change the heat generated within the resistor. As a result, the temperature of the heated resistor and its related resistance Rh are modified such that the signal voltage Vs is equalized with the reference voltage Vr. Under these circumstances, the bridge voltage Vb is indicative of the amount of fluid flow.
In a flowmeter of the above kind, it is desirable that the bridge voltage Vb be temperature compensated over the ambient temperature range. This means that for any given amount of flow, the change in bridge voltage Vb is controlled in a prescribed fashion as the ambient temperature changes. For instance, it may be desirable that the change in bridge voltage Vb be zero over the ambient temperature range. Alternatively, it may be desirable that the bridge voltage Vb change in a predetermined manner over the ambient temperature range so as to cancel or compensate for some inverse temperature responsive change that would be otherwise be induced in the bridge voltage Vb. An example of the latter situation might be where the bridge voltage Vb is fed through a subsequent voltage conditioning circuit that produces an output which is a temperature responsive function of the bridge voltage.